Hydrogen can be stored in the forms of a solid, liquid or gas, but a solid-phase hydrogen storage technology which stores hydrogen in the form of a solid material is preferable in terms of stability and efficiency.
Magnesium hydride (MgH2) has a theoretically high hydrogen storage capacity of 7.6% by weight and thus, it is an attractive candidate as a solid-phase hydrogen storage material of a high capacity. However, there exists the problem that the rate of MgH2 formation (storage) and decomposition (emission) are extremely low. It has been reported in early 2000 that if a transition metal or an oxide thereof is added as a catalyst into magnesium hydride, the rates for hydrogen storage and emission become rapid (U.S. Pat. Nos. 6,572,881 and 6,752,881), which induces active studies on magnesium hydride as a hydrogen storage material.
The conventional method for adding the catalyst to magnesium hydride is conducted by pretreating a magnesium hydride powder by high energy ball milling for several tens of hours, subsequently mixing a transition metal powder or an oxide thereof with the ball-milled magnesium hydride powder, and then, conducting high energy ball milling of the powder mixture for several tens of hours or more [W. Oelerich et al. “Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials,” Journal of Alloys and Compounds, 315, 237-242 (2001)].
However, the above method has problems in that a) it requires two steps of a complicated high energy ball milling process, b) the productivity is low due to the excessively long high energy ball milling process time, and c) the probability of incorporating impurities during high energy ball milling is high.